Field of the Invention
1. Technical Field
The present invention relates to x-ray lasers and compact high power pulse forming line drivers for x-ray lasers and methods for producing short wavelength x-ray laser pulses.
2. Background Art
Future requirements for weapon effects testing will necessitate improvements in the current radiation source capability for soft x-rays. Furthermore, as man stretches the micro-limits of technology, shorter wavelength optical sources become a major limitation in the ability to fabricate more dense electronic circuits, look at smaller objects, or even break apart smaller building blocks. Although x-ray lasers were first demonstrated in plasmas generated in large laser facilities in 1984, widespread use of these sources have been limited due to their large size, high cost, and complexity.
As most of the markets require intense or high brightness source levels, there are two sources that are presently commercially available: synchrotrons and laser plasma sources. Synchrotrons cost about $25,000,000 with a facility cost of about $30,000,000. The laser based sources run from $800,000 to $3,000,000. Furthermore, neither of these two sources are coherent or provide a narrow line width or beam width.
Advances in many areas of effects simulation and optical processing require a radiation source which cannot be currently provided by any single available source, much less a table-top source. Simultaneous requirements for short wavelength (&lt;500 .ANG.), high intensity, narrow line, good collimation, small beam size, short pulse length, coherence, and high repetition rate have slowed or prevented advances in these fields.
Other approaches to develop a soft x-ray laser include line focusing a laser beam on a target, such as carbon, and achieving lasing in the recombining carbon plasma. Park, C. O., Polonsky, L., Suckewer, S., "Recent Results on Development of a Table-Top Soft X-ray Laser", Appl. Phys. B, vol. 58, p. 19, 1994. Imploding plasmas created by water pulse line machines have created the high plasma caricatures necessary to produce x-ray radiation, but the machines used to produce the plasma are very large and do not utilize the radial transmission line or Blumlein technology taught by the present invention.
Direct excitation of a plasma by a pulsed discharge may result in efficient x-ray production, but early efforts centered around very large pulsed power machines and suffered from plasma nonuniformity and instabilities. Rocca, et al, proposed (Rocca, J. J., Cortazar, O. D., Szapiro, B., Floyd, K., and Tomasel, F. G., "Fast-Discharge Excitation of Hot Capillary Plasmas for Soft X-ray Amplifiers", Physical Review, vol. E47, p. 1299, 1993) a scheme which promised to overcome these limitations by using fast discharge excitation of capillary channels to pump collisionally excited lasers. Rocca, et al, successfully demonstrated this scheme (Rocca, J. J., Shlyaptsev, V., Tomasel, F. G., Cortazar, O. D., Hartshorn, D., and Chilla, J. L. A., "Demonstration of a Discharge Pumped Table-Top Soft X-ray Laser", Physical Review Letters, vol. 73, p. 2192, 1994; Rocca, J. J., Tomasel, F. G., Marconi, M. C., Shlyaptsev, V. N., Chilla, J. L. A., Szapiro, B. T., and Giudice, G., "Discharge-Pumped Soft X-ray Laser in Neon-Like Argon", Phys. Plasmas, vol. 2, p.2547,1995; and Rocca, J. J, Marconi, M. C., Chilla, J. L. A., Clark, D. P., Tomasel, F. G., and Shlyaptsev, V. N., "Discharge-Driven 46.9-nm Amplifier with Gain-Length Approaching Saturation", IEEE Journal of Quantum Electronics, vol. 1, p.945, 1995) using an argon plasma generated in the devices shown in FIG. 1. This plasma lased at the 46.9 nm, J=0-1 line of Ne-like argon. The Marx generator (1) pulse charges the capacitor (7) formed by the two plates. The switch (2) is in series with the capillary discharge (3). When the switch fires, the energy stored in the water capacitor (7) is conducted as current through the switch (2) current through the capillary discharge (3) to pump the gas and produce x-ray lasing.
Rocca, et al, discovered that very fast risetime discharges are required for efficient x-ray laser operation, yet in the gain experiments they used capillary lengths of up to 12 cm. They used a discharge capacitance of 3 nfd. The problem of trying to drive long discharges is that the inductance increases linearly with the discharge length. The inductance of a coaxial discharge is given by: ##EQU1## where l is the length of the discharge, r.sub.2 is the outer radius of the current return, and r.sub.1 is the radius of the discharge channel. If the current return radius is 2 cm and the discharge channel radius is 100 microns (note that the results are not very sensitive to these assumptions because of the logarithmic scaling), for a 1 cm long discharge, the inductance is 10 nH, which would result in an inductance limited current risetime of about 9 ns. A 12 cm discharge will result in an inductance 12 times greater (120 nH) for an inductive risetime limit of 30 ns. The pulse width changes from about 20 ns to over 60 ns by changing the discharge length. Certainly an unfavorable way to scale the laser if small current risetime is essential for efficient operation.
In addition, significant losses are apparent in the Rocca, et al, system. A typical current waveform is reproduced in FIG. 2. This is generated from the driver shown in FIG. 1. Note that significant ringing of the current waveform indicates poor matching between the circuit and the discharge. Since Rocca, et al's results indicate that lasing occurs primarily in the first current cycle, the energy ringing during the other cycles represents a significant system inefficiency. Furthermore, a ringing circuit places significant stresses on the system and is known to reduce the lifetime of such systems.
By analyzing the circuit and the wave forms we find that the inductance is about 39 nH and the charge voltage is about 430 kV which is substantially less than the 700 kV Marx voltage. The 39 nH is also substantially more than the coaxial discharge inductance calculated above (10 nH) and is due to other contributions to the inductance, especially the switch inductance.
The estimated resistance is shown in FIG. 3 and the voltage is shown in FIG. 4. And, by knowing the voltage and current, the time dependent power can be derived as shown in FIG. 5. If the power curve is integrated, the energy delivered by Rocca et al's pulsed power system is about 220 joules compared with 735 joules stored in the Marx bank. Furthermore, not all of this energy ended up in the laser discharge because the switch voltage drop dissipated some portion of the energy. Since the voltage drop (losses) in a gas discharge switch are proportional to the molecular weight of the switch, and the molecular weight of the switch gas was about 4 times the weight of argon, the switch losses may have been comparable with the argon discharge energy (assuming that the switch channel length was about 2.5 mm in length). That is, only about 100 joules may have gone into the argon discharge resulting in a peak power input of about 5 Gwatts and a peak voltage of only about 200 kV.
To summarize, there are at least four problem areas which will limit the future of this capillary discharge technology unless solved:
1. Poor power scaling due to discharge inductance;
2. Low energy transfer efficiency (&lt;15%);
3. Factor of 3.5 voltage drop in the system; and
4. Ringing discharge reduces system lifetime.
The radial transmission line has been applied to the switched power linac as a means of creating high particle acceleration for low capital cost. The basic principles of operation and analysis have been developed for these applications and small scale laboratory tests have been conducted on the radial transmission line performance. However, the present invention is the first combining the radial pulse forming line, in either transmission line or Blumlein configuration, with a capillary discharge to produce an x-ray laser.
Finally, there are three types of lasers: a) those using a resonant cavity, b) the single ended mirror laser cavity, using amplified spontaneous emission and c) the mirrorless laser. The resonant cavity is formed by two mirrors with a gain region between them. One of the mirrors is semi-transparent. The light is reflected multiple times between the mirrors, with some of the light leaking out of the semi-transparent mirror. This type of cavity requires high reflectivity from the mirrors to avoid high losses. The single ended cavity is similar to the resonant cavity with the semi-transparent mirror removed. The light can traverse the gain region at most twice with this configuration. This laser removes the requirement for a semi transparent mirror, for which materials technology may not be available in certain circumstances, such as x-ray lasers. This laser requires high gain from the gain medium to provide efficient power extraction from the gain medium. The mirrorless laser does not use any mirrors, but achieves geometric confinement of the beam by the geometry of the gain region. This laser requires higher gain than the single mirror laser because of the lack of light reflection and multiple passes through the gain medium. This laser results in light output from both ends. This laser is very attractive for x-ray lasers because of the difficulty of fabricating mirrors at soft x-ray wavelengths.